SLA 3D Printing Technology : Detail Explanation

The SLA or Stereolithography technology also referred to as optical fabrication, photo-solidification, or resin printing is a part of VAT Polymerization method which uses liquid polymer resin to build fine 3D objects by curing them with a UV laser beam.

There is another technology which is almost similar to SLA technology called DLP (Digital LightProcessing) technology. But we will now learn about the SLA technology and the difference between the SLA and DLP will be explained in later article.

In SLA 3D Printing the liquid resin is filled in a tank and a UV laser beam is projected from top on the surface of the resin covering the area as per the slice created by the 3D printer software. The UV laser cures the whole layer point by point with accuracy and fine surface finish.

SLA technology uses the photo polymer resin which consist of monomers and oligomers which are cross-linked together so that they can perform photo-chemical process. These polymers get solidifies when exposed to UV light with 405 or 365 nanometer wavelength.

World's First SLA 3D Printer by Chuck Hull of 3D Systems

The history tells us that the first 3D Printing technique developed by ChuckHull of 3D Systems in 1986 was the “SLA Technology” which uses VAT Polymerization method for printing. 3DSystems was the first company who commercialized this technology back in 1986. After that many industries like automobile and mechanical started using this technology as a prototyping machine or model making machine. Engineers can create their prototypes in more effective manner and they can print faster and cheaper compare to traditional method.

Since last decade many different companies started to manufacture SLA 3D Printers as the patent expired in 2009. Companies like DWS, FormLabs started commercializing their own SLA 3D Printers which works under same process but the formation or we can say the printing procedure was different.

Pre-Printing Process:

Every 3D Printing process starts by developing the 3D model in the 3D design software called the CAD (Computer Aided Design) file. This file is then converted into .STL file also known as 3D Printer file.

This STL file is then repaired using software like Materialise Magics or AutodeskNetfabb which is highly needed in VAT polymerization technique. For example, in some cases the file is damaged or have empty shells than its recommended to repair the file else that part will not get printed in the 3D printer and there are 90% chance of printing failure.

Supports are added to the STL file as per requirement of the product. These supports are mandatory as the object will need some ground support otherwise the part which hangs will not get printed and due to that the entire print will be failed.

The supported STL file is then uploaded into the printer program where it is cut into slices as per the micron we set. For example, if we set 50 microns in printing parameter than the program will divide 1 mm hight into 20 slices, which means the thickness of each slice will be 0.05 mm.

SLA 3D Printing Process:

Once the pre-processing is done its time for the 3D Printer to print them layer by layer. Once the printer is turned on it will check if all the sensors are in working condition or not.

Then the perforated metal platform will travel downwards and will be dipped inside the tank in which photo polymer resin is kept. It will be dipped one-layer height below the surface of the resin.

The laser beam will hit the surface of the photo polymer resin and draw the first layer as per the slice we made. The laser will solidify each part it hits the liquid.

After solidification of the resin the platform moves one layer down and a metal blade known as the “Recoater” moves on top of the printed layer and cleans the uncured resin part. This process is repeated until the whole product is completed.

The time required to complete one platform depends on the size and configuration of SLA 3D printer. The small dimensional items take less time compared to large dimensional items.

Post Process:

Once the entire printing process is completed and the product is built, the platform rises up and all the extra resin gets drained through the perforated metal plate. The built part is then removed from the platform with sharp edge spatula as it got stick and hard due to UV laser curing process. The part is still green and can brake easily so we have to be care full while handling the part.

After removing it from the plate the part is dipped into the alcohol for several times to remove extra unwanted resin from the part surface. The part is then dried completely with the help of air blower and then the supports are removed with the help of cutter. One should be careful while removing the supports as in some parts where the thickness is minimum the parts get damaged.

Once the supports are removed its placed inside UV oven for final curing. In UV oven the part is put between the UV lights for 10 to 15 minutes depending upon the resin and thickness of the part. Here the built part gets the highest possible strength and become more brittle.

After taking out the part from oven if needed extra process like sanding or brushing can be done to remove the support marks from the surface of the part.

SLA Materials:

Materials that are used for SLA printers are costly compared to the FDM materials as they comprise of oligomers and monomers which undergoes chemical process. The process of making the liquid photo polymer is expensive and the product made by these materials are also brittle compared to FDM materials. So, the SLA 3D printing process is mostly used where direct casting technique is used.

Facts about SLA 3D Printing:

3D Systems first 3D printer was the SLA-1 which was the basic model of SLA technology. Later, they invented a new machine called the VIPER SLA which was highly efficient and can print high quality parts with accurate dimensions.

SLA is the most accurate and precise technique with high quality surface finish with high dimensional accuracy. It can print upto 25 micrometre layer height, but the time required to print the part will be increased.

The first SLA 3D printer made by Chuck Hull in 1986 was called the “Top-Down Technology”. In this technology the light source is kept above the resin tank and built platform. The UV Laser light focus from top and the object is printed on top of the built platform. So, there is no peeling effect and the part needs less supports.

There is an alternate technology which is completely inverse to top-down technology. This technology is called the “Bottom-Up Technology”, in this the light source is below the transparent resin tank and the built platform is fixed upside down above the resin tank. The UV Laser light gets focused on the transparent resin tank from bottom and draws the image after which the platform moves upward and a peeling effect is generated which separates the cured part from the liquid resin similar to recoater blade in top-down technology.

There are popular companies like Formlabs and DWS Systems who has developed the bottom-up technology in SLA 3D printing. The bottom-up SLA printers are less expensive compared to top-down technology because it has fewer moving parts and no metal parts except the metal-built plate.

The VAT Polymerization technology is widely used in Dentistry for making dentures, aligners, night guards and prosthetic, in Jewelry Manufacturing for making direct castable parts by eliminating mold and wax pieces, in Mechanical Industry for prototyping of new components.

Limitations of SLA 3D Printing:

The SLA technique is slow compared to FDM technique so it will take more time to print.

Extra supports are required if the part has overhangs and slopes otherwise the such parts will collapse during printing process and the entire product will be damaged.

The parts built by liquid resin have less mechanical properties so they cannot be used as an end product.

Extra care needed while working with photo polymer resin. It should be covered with UV protective glass when not used. It gets cured if comes in contact with sunlight or any light containing UV rays.

The parts made by SLA printing degrades over time.

SLA 3D Printing is an expensive process in terms of material, machine and post processing equipments.